In response to the fulfill the ever demanding requirements of ultra-precision micromachining system, a hydrostatic bearing, that is designed to use nothing more than a thin film of fluid or gas to support its load, resulting very small friction between the outer casing of the bearing and the shaft it supports, is becoming the essential component for more and more high-accuracy machine tools.
Comparing with the aerostatic bearing, the hydrostatic bearing is superior in its rigidity, damping performance and ability to eliminate vibration in a milling process, that in addition to its micro/nano-scale positioning accuracy in precision movement, make the hydrostatic bearing ideal for high precision applications such as ultra-precision milling machine. Nowadays, almost every high-accuracy machine tool in the world adopts the hydrostatic bearing system in its guide rail system.
Generally, a typical hydrostatic bearing unit for machine tools with constant-pressure oil supply system is composed of a hydrostatic bearing, an external oil supply and at least one restrictor, in which the hydrostatic bearing can be a hydrostatic journal bearing that is also known as the hydrostatic radial bearing as it is designed for supporting radial loads.
The hydrostatic journal bearing units that are current available on the market to be used in machine tools can be divided into two categories, which are the type without flow compensation and the type with flow compensation.
For the type of hydrostatic journal bearing units without flow compensation, such hydrostatic journal bearing unit is primarily composed of a hydrostatic journal bearing and a restrictor, and that is constructed without oil conduits in fluid communication between two oil chambers that are disposed opposite to each other for flow and pressure compensation. Such hydrostatic journal bearing unit without flow compensation can typically be exemplified by a hydrostatic spindle bearing assembly disclosed in TW Pat. No. M320617, which is composed of: a detachable open-ended cylinder, disposed inside a spindle base of a platform while having at least one bearing configured therein so as to be provided for a spindle to mounted thereat; and a plurality of pressure pouches, disposed inside the bearing and each being configured with a pressure sensor and a nozzle. Accordingly, by connecting each nozzle to a pressure intensifier by a channel while connecting each pressure sensor a controller for controlling all the pressure intensifiers, the spindle can be maintained to rotate without bias in a self regulating manner with very fine tolerance control.
However, since the self regulating of this type of hydrostatic journal bearing units is not enabled by the flow compensation mechanism, but is enabled through the restrictors, it is conceivable that radial loading capacity as well as the rigidity of the aforesaid hydrostatic journal bearing might not be satisfactory so that such hydrostatic journal bearing can only suitable of low-load applications, but not for middle-load or even high-load applications.
For the type of hydrostatic journal bearing units with flow compensation, such hydrostatic journal bearing unit is constructed with oil conduits in fluid communication between two oil chambers that are disposed opposite to each other for flow and pressure compensation in addition to the assembly of a hydrostatic journal bearing and a restrictor, and thereby, its radial loading capacity and rigidity are improved. There are different designs for the hydrostatic journal bearing units with flow compensation. One of which is the design having the oil chambers formed on the axis of its spindle, as the one disclosed in U.S. Pat. No. 5,700,092. However, in the aforesaid design, since the formation of the oil chamber can be restricted by many factors such as the size of the axis, the design of the oil chamber is restricted in shape and size. Another design will have the oil chamber to be formed on the sleeve of the spindle, as the one disclosed in U.S. Pat. No. 5,281,032. Since the design freedom of the oil chamber is comparatively higher due to the fact that it is ease to alter the dimension specification of the sleeve, this type of design is most common in the hydrostatic journal bearing units available today.
In detail, in the U.S. Pat. No. 5,700,092, there are a plurality of pressure supply grooves formed on the sleeve of the spindle and correspondingly, there are a plurality of collector grooves formed on the surface of the spindle that are equally spaced around the spindle with circumferential arc length on the order of 60 angular degrees. Moreover, single or multiple groove pocket shapes are circumferentially spaced and equal in number to the number of collector grooves and axially displaced from the collector grooves, and thereby, flow channels are provided either by holes drilled in the collector grooves through chords of the spindle traversing the same to connect to one corner of the pocket, or by surface grooves formed along the external spindle surface to traverse the same, such that when fluid flows axially from the pressure grooves across the spindle into the collector grooves, in proportion to the radial clearance between the spindle surface and the bore, it can flow to the pocket opposite to the collector, and thus act to provide a restoring force in proportion to the radial displacement of the spindle. Nevertheless, although the aforesaid disclosure can provide a new and improved self-compensating hydrostatic journal bearing, it is disadvantageous in its high manufacturing cost resulting from its complex structure and the machining to the sleeve and the spindle for forming those corresponding grooves. Moreover, since the support rigidity of the spindle in the aforesaid disclosure is increased with the increasing of its rotation speed, the hydrostatic journal bearing of the aforesaid disclosure is not suitable for middle-load and low-load applications.
On the other hand, in the U.S. Pat. No. 5,281,032, circular annular pressurized-fluid-receiving grooves provided in the opposed housing bearing surfaces are used for regulating the fluid fed to longitudinal recess pockets formed in the opposing bearing surfaces, at an angle to avoid the occurrence of turbulence, to provide a thin film or layer of pressurized fluid in the gaps between a shaft and the opposing bearing surfaces; the regulation establishing differential pressures in the opposing bearing surface pockets to compensate for loads applied to opposite sides of the bearing. Although the hydrostatic journal bearing of the aforesaid disclosure is also self compensating, it is only suitable for the type of bearings whose spindles are only capable of unidirectional rotations since its pressurized-fluid-receiving grooves are connected to longitudinal recess pockets formed in the opposing bearing surfaces at an angle, and thus the usage of such hydrostatic journal bearing is limited.
Therefore, it is in need of a hydrostatic journal bearing with self-compensating ability that is low in cost and high in reliability.